The Metabones Speed Booster adapter brings the full frame look to Sony mirrorless APS-C and Super 35mm E-mount. (Indeed it will bring the Super 35mm look to Micro Four Thirds but not until March).
It also gives you an extra stop of light. F2.8 becomes F2.0 and F2.0 becomes F1.4. F1.2 becomes a lightning fast F0.90.
It would appear the full frame sensor is now rather pointless. Is there a catch, a trade off in image quality?